MySQL 8.0

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create table MajorTable2
( IdNum int not null
, NameEmployee char(1) not null
, IdInsurance int
, num_employees_not_insured int);
insert into MajorTable2 (idNum, NameEmployee)
values (1, 'a'), (2,'b'), (5, 'c');

Records: 3  Duplicates: 0  Warnings: 0


insert into MajorTable2 (idNum, NameEmployee, IdInsurance)
values (1, 'a', 1), (2, 'b', 1);

Records: 2  Duplicates: 0  Warnings: 0


WITH T (n) as (
    SELECT count(*) - count(IdInsurance) from MajorTable2
)
update MajorTable2 set num_employees_not_insured = (
    SELECT n from T
);

Rows matched: 5  Changed: 5  Warnings: 0


select * from MajorTable2;


IdNum
NameEmployee
IdInsurance
num_employees_not_insured


1
a
null
3

2
b
null
3

5
c
null
3

1
a
1
3

2
b
1
3


UPDATE MajorTable2 AS m 
CROSS JOIN (
    SELECT COUNT(*) - count(IdInsurance) AS cnt 
    FROM MajorTable2
    
) AS g 
    SET m.num_employees_not_insured = g.cnt;

Rows matched: 5  Changed: 0  Warnings: 0


select * from MajorTable2;


IdNum
NameEmployee
IdInsurance
num_employees_not_insured


1
a
null
3

2
b
null
3

5
c
null
3

1
a
1
3

2
b
1
3